Application of x-ray absorption spectroscopy to oxygen(2)-reactive metalloenzymes and inorganic complexes.

摘要

Aerobic life depends on transition metals (Mn, Fe, and Cu) for all stages of the capture, carriage, and processing of O2. Metal/O2 chemistry is therefore a subject of fundamental biological importance. This thesis describes a series of problems in metal/O2 chemistry where a structural approach was of particular consequence. The structural tool used is x-ray absorption spectroscopy (XAS), a scattering technique which provides local (within ≤6 A radially) structural and electronic information about transition metals in crystalline/noncrystalline solid or solution states. XAS was used to characterize the transiently stable products of the reaction of several Cu(I) and Fe(II) complexes with O2. In each case, two or three metal complexes react with a single molecule of O2 and reduce it by two or more unconventionally four electrons. The four-electron reduction of O2 by two Cu(I) complexes was shown by XAS to generate Cu(III). In general, the peroxide- or oxide-level products serve as models for comparison to activated O2 in multinuclear metalloenzymes. XAS was also used to characterize two O2-activating metalloenzymes: the Cu-containing particulate methane monooxygenase (pMMO) and the Fe-containing soluble methane monooxygenase (sMMO). Though both enzymes catalyze the NADH-dependent reaction of methane and O2 to produce methanol and water, they are compositionally distinct and are found in different locations within the same organisms. The cytosol-soluble sMMO is well characterized. The XAS of the oxidized form of sMMO (from Methylococcus capsulatus (Bath)) and of several binuclear iron models of its active site are included in this thesis. The membrane-bound pMMO is far less well characterized. An XAS characterization of pMMO (from Methylococcus capsulatus (Bath)) under several conditions---with copper fully reduced, fully oxidized, in the presence of suicide substrate, etc.---is reported here. Finally, attention is paid throughout the thesis to the XAS methodology, including a basic characterization of the Cu(III) K-edge as well as several detailed explanations of how XAS data are analyzed to yield structural information.